A light emitting diode (LED) is a solid state device that converts electrical energy to light. Light is emitted from active layers of semiconductor material sandwiched between oppositely doped layers when a voltage is applied across the doped layers. In order to use an LED chip, the chip is typically enclosed in a package that focuses the light and that protects the chip from being damaged. The LED package typically includes contact pads on the bottom for electrically connecting the LED package to an external circuit. Conventionally, an LED chip is designed to be packaged either as a discrete light emitter or with a group of LED chips in an array. The LED chip of the discrete light emitter is typically mounted on a carrier substrate, which in turn is mounted on a printed circuit board. The LED chips of the array, however, are typically mounted directed on the printed circuit board without using the carrier substrate.
Array products are not conventionally made using the discrete light emitters as building blocks. The carrier substrate of the discrete light emitter is typically considered needlessly to occupy space on the printed circuit board under an array. Moreover, conducting through-hole vias through the carrier substrate of the discrete light emitter would have to be reconfigured in order to connect properly to contact pads on the printed circuit board for each new array design. Thus, no carrier with a particular set of through-holes vias could be used as a standard building block. The problem of the through-hole vias in the discrete emitters can be solved by electrically connecting the LED chips to traces and contact pads on the top side of the carrier substrate. But eliminating the through-hole vias by connecting the LED chips to pads on the top side of the carrier substrate creates the new problem of how to connect the pads to a power source because the carrier substrate is no longer electrically coupled to the printed circuit board below.
FIG. 1 (prior art) shows an existing array product 10 with an array of twenty-four LED chips electrically connected to pads 11 on the top side of a carrier substrate 12. Array product 10 is the XLamp® MP-L EasyWhite product manufactured by Cree, Inc. of Durham, N.C. In FIG. 1, carrier substrate 12 is mounted on a metal disk 13 as opposed to on a printed circuit board. Carrier substrate 12 is attached to metal disk 13 using thermal glue 14. Array product 10 is inelegantly connected to power by hand soldering individual wires of the positive 15 and negative 16 power cord leads to the pads 11. Array product 10 has no features that facilitate connecting the pads 11 on the top side of carrier substrate 12 to a power source in the board or plate below. And array product 10 is not configured to be incorporated into a group of array products.
When LEDs are packaged in arrays as opposed to as discrete light emitters, the LED chips of the arrays are mounted directly on a printed circuit board without the carrier substrate conventionally used with discrete light emitters. The LED chips packaged as arrays are electrically connected to contact pads and to traces in a top trace layer of the printed circuit board. The LED chips are wire bonded to the traces on the top side of the printed circuit board. The printed circuit board is then segmented to form discrete array light sources. Larger exposed areas of the traces on the top side form contact pads to which supply power is connected to each discrete array light source.
The LEDs are typically covered with a layer of phosphor before the array light sources on the printed circuit board are segmented or singulated. The phosphor converts a portion of the blue light generated by the LEDs to light in the yellow region of the optical spectrum. The combination of the blue and yellow light is perceived as “white” light by a human observer. Before the array light sources are segmented, the LEDs are typically covered by a layer of silicone that is formed into a lens above each light source. The layer of silicone also protects the LED chips and top-side wire bonds.
A slurry containing the phosphor has been conventionally dispensed manually into a ring or dam around the LED chips of each array light source. Then injection molding or casting molding has been used to form a lens above each array light source. The manufacturing process for LED light sources has been improved by combining the steps of dispensing the phosphor and forming the lens. By adding the phosphor to the silicone, the separate step of dispensing phosphor can be eliminated, and lenses are formed with phosphor dispersed throughout each lens. The lenses are formed using injection molding in which lens cavities that contain the LED dies are filled with the lens material, and the excess lens material is squeezed out of a leakage path.
When casting molding is used, a phosphor silicone slurry is first dispensed into the bottom half of each cavity, and then the top half of the cavity closes to define the lens structure and squeezes out the excessive lens material. The injection molding and casting molding processes have multiple disadvantages. First, the phosphor and the silicone are expensive, and the lens material that is squeezed out of the cavities is wasted. Second, the quality of the lenses formed with injection molding and casting molding is low because bubbles and nonuniformities remain in the finished product.
Fabricating an LED lens using these techniques is expensive because there are significant material losses and because non-standard semiconductor packaging technologies and equipment are used to package the lens. Therefore, systems and methods that reduce manufacturing costs by reducing waste and by making it easier to package LED dies/arrays using standard semiconductor packaging technologies and equipment are sought. In addition, systems and methods that enable LED package sizes to be shrunk to smaller sizes and to be handled using semiconductor packaging technologies and equipment are also sought.